All references cited in this specification, and their references, are incorporated by reference herein where appropriate for teachings of additional or alternative details, features, and/or technical background.
Disclosed in the embodiments herein is a method for correcting for internal resistance losses in the secondary windings of a transformer to reduce errors in output voltage from a high voltage power supply.
Certain devices have components that require a voltage high enough to cause corona discharge, a controlled static discharge. This corona discharge is used to charge or discharge a target member such as a photoreceptor belt in a xerographic copier or printer. This provides the component with sufficient instantaneous current density for proper operation, without exceeding a maximum average current value. In such components, if the required current density is greater than the desired average current, a chopped current at an appropriate duty cycle is required.
Some power supplies employ pulse amplitude modulation (PAM) for this type of use, which produces a high voltage pulse at a fixed duty cycle and varies the voltage to obtain the correct average current value. Other power supplies employ pulse width modulation (PWM) and pulse frequency modulation (PFM) all of which are used high voltage applications. In most cases, these switch-mode power conversion schemes are used to improve efficiency and reduce the size of magnetic devices such as transformers.
Among those devices that have such components are “image-on-image” xerographic color printers wherein multiple corotrons must be precision charged and controlled to provide desired print quality. FIG. 1 (prior art) is a simplified “image-on-image” xerographic color printer in which successive primary-color images are accumulated on a photoreceptor belt, and the accumulated superimposed images are in one step directly transferred to an output sheet as a full-color image.
Specifically, the FIG. 1 embodiment includes a belt photoreceptor 10, along which are disposed a series of stations, as is generally familiar in the art of xerography, one set for each primary color to be printed. For instance, to place a cyan color separation image on photoreceptor 10, there is used a charge corotron 12C, an imaging laser 14C, and a development unit 16C. For successive color separations, there are provided equivalent corotron, imaging laser and developer elements 12M, 14M, 16M (for magenta), 12Y, 14Y, 16Y (for yellow), and 12K, 14K, 16K (for black). The successive color separations are built up in a superimposed manner on the surface of photoreceptor 10, and then the combined full-color image is transferred at transfer station 20 to an output sheet. The output sheet is then run through a fuser 30, as is familiar in xerography.
Also shown in FIG. 1 is a set of what can be generally called “monitors,” such as 50 and 52, which can feed back to a control device 54. The monitors such as 50 and 52 are devices which can make measurements to images created on the photoreceptor 10 (such as monitor 50) or to images which were transferred to an output sheet (such as monitor 52). These monitors can be in the form of optical densitometers, calorimeters, electrostatic voltmeters, etc.
Control of voltage to a component or load may be by way of one or more transformer(s). Transformers are magnetic devices consisting of two or more multiturn coils wound on a common core, the coil connected to the energy source being referred to as the primary coil or winding and the coil in which current is induced by the primary coil being referred to as the secondary coil or winding. As understood by those skilled in the art, the turns ratio of the primary coil to secondary coil determines the transformer's voltage ratio, an increase in turns of the secondary coil with respect to the primary coil resulting in a boost of voltage at the secondary.
In precision applications the accumulated errors contributing to output voltage variability, including variation in primary winding voltage, transformer turns ratio and various losses make it necessary to measure or infer output voltage and compare that to the desired output voltage in a closed-loop voltage control circuit. In the most simple topologies sensing resistors in conjunction with a potentiometer may be used at the primary coil of the transformer to measure and control voltage input into the transformer and from that the secondary, output, voltage. Sense elements attached to the low side of the secondary winding of a transformer may also be employed to measure output current and to detect output faults or current overloads.
Determination of output voltage at the secondary side of the transformer by inference of the measured voltage at the primary side may be designed and adjusted to properly compensate for the first two of three listed error contributors. Such determination may include inaccuracies due to internal resistance (IR) losses in the secondary windings (which typically comprise many, many windings of small diameter coils) as well as the IR losses associated with other external secondary resistances that may be included to protect the HV power source and load in the event of an arc or other fault condition. Correction of all such errors may be made by additional sense windings or the incorporation of divide down resistors on the secondary side, but these measures significantly add to expense and complexity of design. Errors in output voltage sensing may significantly degrade the accuracy of very sensitive devices. For example, “image-on-image” printing devices require precise control of high voltage sources to achieve outstanding print quality.
It would therefore be useful to permit primary side voltage sensing and regulation without need for secondary side voltage sensing. And to successfully do this, it may necessary to properly compensate for secondary IR loss in the primary side voltage sensing circuitry.